The present invention is directed to a magnetic resonance device with a gas container having an alkali metal hydride coating on the inner surface of the cell. The use of a container having its rotationally symmetric axis oriented at a given angle to the magnetic field, the container enclosing at least one magnetic moment gas having a nuclear electric quadruple moment, included here as part of the preferred embodiment of the invention, is the subject of a separate patent application, GCD 80-27-B, filed by T. M. Kwon and C. H. Volk, concurrently with the present application.
Magnetic resonance phenomena are well understood by those of ordinary skill in the art and a variety of its practical applications in the science and engineering fields are readily available. For the purposes of this discussion, magnetic resonance includes both atomic magnetic resonance and nuclear magnetic resonance.
One particular and important application of the invention to be described is to a nuclear magnetic resonance (hereinafter refered to as NMR) angular rate sensor or gyroscope. U.S. Pat. No. 4,157,495, hereby incorporated by reference into this document, discloses a NMR gyroscope that operates on the principle of sensing inertial angular rotation rate or angular displacement about a sensitive axis of the device as a shift in the Larmor precession frequency or phase, respectively, of one or more isotopes that possess nuclear magnetic moments.
The gyroscope is composed of an angular rotation sensor and associated electronics. The principal elements of the sensor are a light source, an NMR cell, a photodetector, a set of magnetic shields and a set of magnetic field coils. The principal elements of the elctronics are signal processing circuits for extracting the Larmor precession frequency and phase information as well as circuits for generating and controlling various magnetic fields, both steady and varying sinusoidally with time, that are necessary for the proper operation of the device.
The NMR cell is mounted within a set of magnetic shields in order to attenuate external magnetic fields to acceptable low levels. Magnetic field coils are used to apply very uniform magnetic fields to the NMR cell. Both a steady field and an ac carrier field are applied along the sensitive axis of the device and AC feedback fields are applied along one of the transverse axes. The DC magnetic fields along both transverse axes are controlled to be substantially zero. The NMR cell contains a single alkali metal vapor, such as rubidium, together with two isotopes of one or more noble gases, such as krypton-83, and xenon-129, or xenon-131. One or more buffer gases such as helium and nitrogen may also be contained in the cell. The NMR cell is illuminated by a beam of circularly polarized light that originates from a source such as a rubidium lamp and which passes through the cell at an angle with respect to the steady magnetic field. Absorption of some of this light causes the atomic magnetic moments of the rubidium atoms to be partly aligned in the direction of the steady magnetic field. This alignment is partly transferred to the nuclear magnetic moments of the noble gases, and these moments are caused to precess about the direction of the steady magnetic field, which in turn creats magnetic fields that rotate at the respective Larmor precession frequencies of the two noble gases. These rotating fields modulate the precessional motions of the rubidium magnetic moments, which in turn produce corresponding modulations of the transmitted light, thereby making it possible to optically detect the Larmor precession frequencies of the two noble gases.
The modulations of the light intensity are converted into electrical signals by a photodetector, and these signals are then electronically demodulated and filtered to provide signals at the Larmor precession frequencies of the two noble gases. The difference between the two precession frequencies is used to accurately control the steady magnetic field so that it is constant. One of the noble gas precession frequencies is subtracted from a precision reference frequency. The resulting difference frequency is a measure of the angular rotation rate of the gyroscope. The magnitude of an individual nuclear magnetic moment is extremely small and the natural equilibrium condition is one in which a nearly random orientation of moments exists in an ensemble of atoms. Techniques must be used to orient a significant fraction of these magnetic moments in a single direction so that a macroscopic magnetic moment, and consequently a measureable signal, will be produced.
The aligned magnetic moments of the alkali metal system and of both noble gas systems of atoms are subject to relaxation mechanisms which cause their alignments to decay exponentially with time towards their natural equilibrium condition of random orientation. Each system of moments is characterized by a relaxation time constant which depends on the kinds and quantities of all other constituents and upon the total environment in the NMR cell. The steady state fractional alignment of each system of moments is a function of both the pumping rate and the relaxation time for the system, with larger fractional alignments, hence larger signal amplitudes, being achieved when the relaxation times are also long.
Accordingly, a number of prior art techniques exist to achieve longer relaxation times. In one of the techniques, a suitable amount of a buffer gas such as helium or nitrogen is also contained in the cell in order to reduce the relaxation effects due to interactions of the magnetic moments with the walls of the cell. In another technique, particular isotopes of particular noble gases are chosen as the nuclear magnetic moment gases specifically for their long relaxation times. However, a problem still exists in that certain, otherwise desirable magnetic moment gases have relaxation times too short to provide a practical device.
In an article by D. S. Bayles, I. A. Greenwood, and J. H. Simpson in an unpublished report entitled, "Noise Sources in NMR Oscillators and Relaxation Phenomena in Optically-Pumped Mercury Isotopes", Final Scientific Report, Air Force Office of Scientific Research, 1976, it was disclosed that the relaxation time constant of the vapor of mercury-201, a species having a quadrupole nuclear moment, is dependent on the particular angle of orientation of the NMR cell to the externally applied magnetic field. The angle between the cell and the magnetic field which yielded the maximum relaxation time constant was termed the "magic angle", a term which will be used herein.
C. H. Volk, J. G. Mark, and B. C. Grover disclose in an article in Physical Review A, Volume 20, pps. 2381-2389, December, 1979, that this angle-dependent effect was observed for krypton-83, a noble gas also having a nuclear electric quadrupole moment. The magic angle was found experimentally to be an undetermined function of the spatial distribution of the reservoir of rubidium metal spread over the cell. Because of this undetermined dependence of magic angle on distribution of the rubidium metal, the magic angle for a given cell must be empirically determined by a time consuming trial and error approach in which the relaxation time constant is measured at many different angles of cell orientation.
It should be noted that no prior art, including the articles referred to above, disclose or suggest how to define the specific reference axis of the cell which must be aligned to the magnetic field so as to yield the magic angle. In other words, there is nowhere disclosed what axis in the cell is to be taken to determine the magic angle. For example, in the article by Volk, et al, the cell axis is arbitrarily defined as "a line that passes symmetrically through the tip off region of the cell as a matter of convenience" (italics supplied for emphasis).